machining pro-tip: measurements and metrology

Metrology Pro-Tips:

always understand the fundamentals of what you are measuring, and what you are measuring it with.

run-out is not error motion. run-out is also almost always a totally useless measurement.

cylindricity is not taper.

your indicator is only as good as its bracket.

your indicator makes a better thermometer than it does a displacement gauge.

reversal should be used whenever possible, and is the only true way to check your measurements besides trust.

don't trust your measurement tools.

question every one of your fundamental assumptions.

nothing is rigid.

nothing is straight.

nothing is flat.

if your axis doesn't repeat, its probably because you measured it badly.


today we spent 12 hours troubleshooting a problem because one of these basic rules was broken and it took that long to figure it out. it seems simple, but its oh so easy to overlook.

oh, and always to add for the folks working on on the shelf machines: your machine is NEVER as good as the spec sheet said.

You forgot "Check if it is plugged in.". :-)

oh another one: watch out for kinematic over constraints. yes overconstraing something is stiff, but if there are any dynamics (ANY dynamics, including vibration), it will be unpredictable. less stiffness in a properly constrained system is better than over stiff in a an over constrained system.

Going down the list you posted, I'm trying to recount in my head whether or not any of my typical students would even know what you're referring to. Some of the pre-engineering people would be on track, but most take a while to even get past common technique on using a set of calipers, micrometers, depth micrometers, height gauges, telescopic gauges, etc. Some people are better than others, and some people are more vested than others.

Of all the biggest measuring problems I see people encounter, it's almost always a case of somebody not understanding how to actually use the measuring tool. 95% of the time it's a set of calipers that hasn't been zeroed correctly. I rarely see people use micrometers unless they already work in industry and know how to use them (or unless we "strongly suggest" them to use them). We also have a set of "standards" gauges that can be used to check any tools being used in the lab....but people just don't use it. I'm not talking about a tool being off by a couple "hairs" (lol) but truly a set of calipers might be off by like 0.015" or maybe even more. and no one is any the wiser.

Some people have a long way to go. that's all...

always a good thing to have your gauge blocks checked too, if that is what you are calibrating your shop tools off of. they float, i used to work in a shop that measured down to the millionth, and they had to have there gauge blocks recalibrated every month. theyd float all over the place on that level.

At what point does radiation or someone listening to the wrong genre of music in the office start affecting your tolerances?

Quote:

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Originally Posted by HipboyScott

At what point does radiation or someone listening to the wrong genre of music in the office start affecting your tolerances?

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youd be surprised how much heat transfer there is with a light source for your microscope ...